This research proposal is a neurophysiological study of the short and long-term effects produced by the climbing fiber afferent system on the processing of mossy fiber afferent input by Purkinje cells in the cerebellar cortex. Acute, decerebrate cat studies will examine the hypothesis that the climbing fiber afferent system augments the responsiveness of Purkinje cells to mossy fiber afferent input. This interaction is postulated to occur in the cerebellar cortex and specific experiments will test this postulate. These studies will also determine whether cerebellar cortical interneurons are involved. The acute cat model will also be used to explore the effects of climbing fiber on neighboring Purkinje cells' responsiveness to mossy fiber input. Another series of experiments will evaluate the relationship between the modulation of Purkinje cell's impulse activity and the activation of its climbing fiber input by a peripheral stimulus. These experiments will not only address the effect of the climbing fiber afferent input on the firing rate of the simple spike activity but also its contribution to the temporal patterning in the simple spike discharge. A semichronic awake cat model will be used to explore the hypothesis that persistent changes can be produced in a Purkinje cell's responsiveness to mossy fiber inputs as a consequence of repeated conditioning with spontaneous climbing fiber inputs as a consequence of repeated conditioning with spontaneous climbing fiber afferent input. The chronic animal studies will also be used to document that the short-term interactions descried in the decerebrate animal are present in the awake, intact animal. Overall these studies will increase our understanding of how the two main afferent systems to the cerebellar cortex, the mossy and climbing fibers, contribute to the output of this structure.